The proposed research is designed to study the effects of low doses of ultraviolet and ionizing radiation on specific culture parameters of human embryos, humans of various ages, and humans with either progeria-like syndrome or Xeroderma pigmentosum will be used. The specific culture parameters to be studied are growth rates, DNA and protein synthesis, in vitro age and transformation. Low doses are defined with respect to plating efficiencies and growth rates. The effect of repeated exposure to low doses over a period of time on irradiated populations will be quantitated with regard to the above mentioned parameters. Similar experiments will be done using cultures maintained in a non-proliferating state. Cell strains with normal and defective DNA excision repair mechanisms will be used to determine the significance of DNA repair on these parameters.